The effect of precipitation of carbonitride on austenite grain growth behavior in Ti-Nb microalloyed steel has been studied. Transmission electron microscopy and optical microscopy were used to investigate the morphology, particle size distribution, and precipitation kinetics of carbonitride, as well as the austenite grain growth behavior.
In as-cast Ti-microalloyed steel slab, Ti-carbonitride particle size became coarser with increasing depth of slab because of slow cooling rate. Dendrite structure of low C slab B was finer than that of high C slab A.
As-cast austenite grain size of slab B which has low C content is finer than that of slab A which has high C content. This is because of the fact that the Δ to γ transformation temperature of slab B is lower than that of slab A. So the austenite grain growth during casting was much restricted in low C slab B. This grain size difference was reflected in reheated condition.
When Ti-microalloyed slab was reheated to a austenite region, carbonitride particle size became larger with increasing depth of slab and reheating temperature. There was no large difference in particle size distribution between low C slab B and high C slab A. But the particle size of slab A was larger than that of slab B at 1300℃ and the austenite GCT(Grain Coarsening Temperature) (i.e.,1300℃) in low C slab B was observed to be higher than that (i.e., 1250℃) in slab A. This is believed to be closely related to the difference of interdendritic segregation arising from a different fineness of dendritic structure.
In an as-cast Ti-Nb microalloyed slab C, the particle size was larger than that in an as-cast Ti-microalloyed slab A and the coarse dendrite shaped particle, which was absent in slab A, was frequently observed.
In slab C, carbonitride particles during reheating were coarser than those in slab A. But the austenite grain size and GCT of slab A and C is not much different.
The coarsening rate of carbonitride in Ti-Nb microalloyed slab C was observed to be faster than that of Ti microalloyed slab A. This is belived to be due to the fact that solute solubility in slab C is higher than that in slab A.
The Precipitation-Time-Temperature(PTT) curve has been determined using solution treated sample of Ti-Nb microalloyed slab C. The precipitation starting time was shortest at 1150℃ and the precipitation rate appeared to be accelerated as a result of Nb addition.